For example, turbine disks, which are heat-resistant members of aircraft engines, power-generating gas turbines, etc., are rotary members that support turbine blades, and are subjected to much higher stress than turbine rotor blades. Therefore, turbine disks require a material excellent in mechanical characteristics, such as creep strength or tensile strength in a high-temperature and high-stress region and low-cycle fatigue characteristics, and forgeability. On the other hand, in order to improve fuel efficiency or performance, an increase in engine gas temperature and a reduction in the weight of turbine disks are required, and therefore the material is required to have higher heat resistance and higher strength.
In general, nickel-based forged alloys are used for turbine disks. For example, Inconel 718 (which is a registered trademark of The International Nickel Company, Inc.) using a γ″ (gamma double prime) phase as a strengthening phase and Waspaloy (which is a registered trademark of United Technoligies, Inc.) using, as a strengthening phase, about 25 vol % of a precipitated γ′ (gamma prime) phase stabler than a γ″ phase are frequently used. Further, Udimet 720 (which is a registered trademark of Special Metals, Inc.) has been introduced since 1986 from the viewpoint of dealing with higher temperatures. Udimet 720 has about 45 vol % of a precipitated γ′ phase and tungsten added for solid-solution strengthening of a γ phase, and is therefore excellent in heat-resistant characteristics.
On the other hand, the structural stability of Udimet 720 is not always sufficient, and a harmful TCP (Topologically close packed) phase is formed during use. Therefore, Udimit 720Li (U720Li/U720LI) has been developed by making improvements, such as a reduction in the amount of chromium, to Udimet 720. However, the formation of a TCP phase still occurs also in improved Udimit 720Li, and therefore the use of Udimit 720Li for a long time or at high temperature is limited.
Powder metallurgical alloys typified by AF115, N18, and Rene88DT are sometimes used for high-pressure turbine disks required to have high strength. The powder metallurgical alloys have a merit that homogeneous disks having no segregation can be obtained in spite of the fact that many strengthening elements are contained. On the other hand, the powder metallurgical alloys have a problem that their production process needs to be highly controlled, e.g., vacuum melting needs to be performed at a high cleaning level or a proper mesh size needs to be selected for powder classification, to suppress the mixing of inclusions and therefore their production cost is significantly increased.
In addition, many proposals have been made to improve the chemical compositions of conventional nickel-based heat-resistant superalloys. All of them contain cobalt, chromium, molybdenum or molybdenum and tungsten, aluminum, and titanium as their major constituent elements, and typical ones contain one or both of niobium and tantalum as their essential constituent element(s). The presence of niobium and/or tantalum is suitable for the above-described powder metallurgy, but is a factor making casting and forging difficult.
Titanium is added for its function of strengthening a γ′ phase and improving tensile strength or crack propagation resistance. However, the amount of titanium added is limited to up to about 5 mass %, because excess addition of only titanium results in an increase in γ′ solvus temperature and formation of a harmful phase, which makes it difficult to obtain a sound γ/γ′ two-phase structure.
Under the circumstances, the present inventors have made a study of optimization of the chemical composition of a nickel-based heat-resistant superalloy and have found that a harmful TCP phase can be suppressed by actively adding cobalt in an amount of up to 55 mass %. Further, the present inventors have found that a γ/γ′ two-phase structure can be stabilized by increasing both a cobalt content and a titanium content so that cobalt and titanium are contained in a predetermined ratio. Based on these findings, the present inventors have proposed a nickel-based heat-resistant superalloy that can withstand higher temperatures for a long time than conventional alloys and that has excellent workability (Patent Literature 1).
Further, some proposals focused on the microstructure of a nickel-based heat-resistant alloy have been made to improve the performance of the nickel-based heat-resistant superalloy (Patent Literatures 2, 3, and 4).
In a nickel-based heat-resistant superalloy produced by powder metallurgy, crystal grains are less likely to become too large even after solution heat treatment performed in a temperature region exceeding a γ′ solvus temperature (at a supersolvus temperature), and therefore crystal grain size and grain size distribution are generally controlled by performing aging heat treatment after solution heat treatment performed in a temperature region exceeding a solvus temperature (e.g., Patent Literature 7). However, while crystal grains are less likely to become too large, it is often the case that the control of crystal grains is poor. Therefore, in order to avoid harmful growth of crystal grains during solution heat treatment performed in a temperature region exceeding a solvus temperature, the importance of strain rate control during forging has also been proposed (e.g., Patent Literatures 5 and 6). Further, in order to promote proper growth of crystal grains, a method has also been proposed in which a nickel-based heat-resistant alloy having a high carbon content is forged at a high local strain rate (Patent Literature 8).
However, the alloys described in the above Patent Literatures are powder alloys whose production process is complicated and production cost is high. The powder alloys vary in optimum microstructure according to their chemical composition, and are therefore considered to be applicable only to some limited materials and production methods.
On the other hand, when a nickel-based heat-resistant superalloy produced by a casting and forging method is subjected to solution heat treatment in a temperature region exceeding a solvus temperature, crystal grains become too large and therefore heat-resistant characteristics are significantly impaired. Therefore, in general, solution heat treatment is performed at 90% or less of a solvus temperature, and then aging heat treatment is performed.
At present, however, no nickel-based heat-resistant superalloy has been found which is produced by a conventional casting and forging method and has heat-resistant characteristics significantly higher than those of nickel-based heat-resistant superalloys produced by powder metallurgy. Therefore, there is a strong demand for development of a nickel-based heat-resistant superalloy that is produced by a casting and forging method capable of significantly simplifying its production process and that is superior also in terms of heat-resistant characteristics and cost to nickel-based heat-resistant superalloys produced by powder metallurgy.
Patent Literature 1: WO 2006/059805
Patent Literature 2: Japanese Patent No. 2666911
Patent Literature 3: Japanese Patent No. 2667929
Patent Literature 4: JP 2003-89836 A
Patent Literature 5: U.S. Pat. No. 4,957,567
Patent Literature 6: U.S. Pat. No. 5,529,643
Patent Literature 7: JP 2011-12346 A
Patent Literature 8: JP 2009-7672 A